This invention relates generally to heat sink assemblies and more particularly to clips used to affix heat sinks to electronic components.
As is known in the art, heat sinks are used in a wide variety of applications, as for example, to remove heat from electronic components. Typically, when the electronic component is a digital component, such as a microprocessor, the component is mounted to a printed circuit board, as for example using ball grid array (BGA) or pin grid array (PGA) technology. The heat sink in mounted in thermal contact with an upper surface of the electronic component. The heat sink generally includes a thermal conductive metal, such as aluminum, formed as a single unit with a plurality of radiating fins or surfaces projecting from a base. The base is held against a surface of the electronic component, either directly, or through an intermediate, thermally conductive interface. A clip, or retainer is typically used to engage the base, or the interface, and force it against the upper surface of the component to thereby ensure a good thermally conductive path between the electronic component and the heat sink. The clip and the heat sink thereby provide a heat sink assembly.
In one form, the clips include cantilever members which deflected upon engagement with the base of the heat sink to provide desired forces. More particularly, one such clip is somewhat M-shaped; the V-shaped portion thereof bridging the pair of legs of the M-shaped clip. The V-shaped portion includes a central, horizontal section with a pair of upwardly sloping sections at the ends of the horizontal section. First ends of the sloping sections join with the ends of the horizontal section. Second ends of the sloping sections join with the pair of legs. These clips are typically fabricated as a planar strip of sheet metal having a proper outline so that, when folded, provide the M-shaped form. Thus, four folds are typically required: Two folds are used to provide the legs (i.e., the folds joining the legs and the second ends of the sloping section) and another pair of folds are used to provide joining of the first ends of the pair of sloping sections and the horizontal section. In order to form these folds in sheet metal, it is necessary to fold the sheet metal beyond its elastic range into its plastic range to obtain a permanent bend, or deformation, in the part. However, because of the characteristics of sheet metal, after it is bent there is a tendency for the material to want to bend, or "spring back" towards its original shape. This tendency is particularly characteristic of shallow, or gently sloping bend, such as bends less than 90 degrees. Thus, this tendency must be compensated for in the bending process, such as by over bending. The distal ends of the legs are dimensioned so that they pass through holes formed in the printed circuit board. The distal ends of the legs are provided with lips to engage underlying surface portions of the printed circuit board forming the peripheral region of the holes. Thus, considering the typical case where the electronic component is mounted to the upper surface of the printed circuit board, a pair of holes is provided through the printed circuit board on opposite sides of the region of the board being occupied by the electronic component. The M-shaped clip is held above the electronic component. As the legs are passed, downwardly, through the holes straddling opposing sides of the component, the horizontal section of the V-shaped portion bridging the legs engages the upper surface of the base of the heat sink. The legs of the clip are forced though the holes overcoming the upward force provided by the surface of the component against the horizontal section of the V-shaped portion (more particularly, by the deflection forces produced by the spring-like, cantilever sloping section) until the lips at the distal ends of the legs engage the bottom portions of the printed circuit board disposed about the holes and thereby the clip locks the base of the heat sink to the upper surface of the electronic component.
While such arrangement may provide an adequate arrangement where the printed circuit board had relatively large holes, as components became more closely packed on the surface of the board, the size and placement of the holes became more precisely defined. Further, the holes sometimes even became obscured. The use of precisely positioned, small holes, placed severe demands on the tolerances required in the manufacture of these clips. The "spring back" effect, plus subsequent operation, make it almost impossible for the clip to hold the desired dimensions. This tendency is particularly characteristic of shallow, or gently sloping bends, such as bends less than 90 degrees. The legs lose their precise vertical orientation requirement and therefore it is difficult to locate them in the holes, particularly where the holes are obscured. Thus, the legs must be maintained in a vertical orientation. Further, because the shape of the clip is not precisely maintained because of these "spring back" forces, the clamping force to be provided by the clip is also adversely effected; for example, the legs may deform if the non-vertical legs are bent together so that they may pass through the holes. This bending together of the legs will result in an upward force on the center section, thereby relaxing the clamping force on the base from the desired value. The clip no longer has a precisely controlled clamping force. Further in this regard, the distance between the horizontal section and the lips used to engage the bottom of the printed circuit board is critical to the amount of force which the clip will provide to the base of the heat sink and the electrical component. Thus, the folding process used to provide the four folds (i.e., and hence provide the cantilever members) must be made such that the legs will pass with proper vertical alignment through the holes and the V-shaped portion made to provide proper engaging force on the upper surface of the base of the heat sink to thereby obtain the requisite thermal conductivity between the component and the heat sink.